The present invention relates to a transgenic plant of the species Solanum tuberosum with a resistance to an oomycete of the genus Phytophthora, to transgenic parts a plant of this type, to a method for its manufacture and to a means for external application to plants.
Even now, potato late blight caused by Phytophthora infestans is still the most prevalent and most economically important potato disease.
Throughout the globe, the pathogen results in loss of earnings, with harvest losses of more than 20 percent. This means that expensive chemical plant protection means have to be used, because the natural defence mechanisms of the potato with the help of which P. infestans is combatted or with which propagation can be slowed down and restricted is not sufficient or not permanent.
Natural plant defence mechanisms, such as the hypersensitive reaction at the infection site, lignification of the cell wall, the production of PR (pathogenesis-related) proteins and the synthesis of phytoalexins are indeed known to contribute to augmenting resistance, but they are always accompanied by an energy loss and thus a loss of earnings for affected plants.
Natural defence mechanisms in plants also include the expression of so-called resistance genes (R genes), the gene products of which interact with microbial avirulence genes (Avr genes) (gene for gene hypothesis) and thus induce a specific defence reaction. This resistance can, however, be interrupted if a pathogen such as P. infestans can dispense with the synthesis of the Avr gene and recognition of the pathogen and thus the subsequent specific defence reaction in the plant host does not occur.
Fire et al. (1998) have already demonstrated that double stranded RNA (dsRNA) can result in the sequence-specific degradation of homologous RNA. Starting from these results, transgenic plants have been developed in the meantime which, with the aid of RNA interference (RNAi) by means of host plant-induced silencing of conserved and essential genes, for example from nematodes or Lepidoptera- and Coleroptera species, can exhibit resistance to these pests in vitro as well as in vivo.
In addition, the host plant-phytopathogenic fungus interaction can constitute an application of the concept of host-induced gene silencing (HIGS) to induce resistance (EP 1 716 238).
Van West et al. (1999) initially used the gene silencing method in Phytophthora, in order to carry out functional analyses of these oomycete-specific genes.
In WO 2006/070227, the use of RNA interference to control fungal pathogens based on contact of dsRNA with fungal cells outside the fungal cell was described for the first time. It proposes a method for the manufacture of a pathogen-resistant plant. In this manner, the RNA interference can be directed against one or more genes of a pathogen as well as several pathogens. Phytophthora infestans is mentioned as a possible fungal pathogen and potato as a possible host plant.
Previous studies have given rise to the hypothesis that host plant-induced gene silencing does not work for every gene and choice of the target gene is essential for functional silencing. Thus, for example, the plasma membrane H+-ATPase PnMA1 in Phytophthora parasitica could not be reduced sufficiently by host plant-induced gene silencing to deliver efficient protection against a pathogen (Zhang et al. 2011). According to this, selection of the target genes is also decisive for effective pathogen defence (Yin et al. 2011).
Recently, a screening system was proposed which was supposed to facilitate the selection of suitable parasitic genes for silencing constructs for the production of pathogen-resistant plants (US 2010/0257634). The identification of appropriate test constructs to induce phytoresistance in potato was also proposed by the authors. In this regard, target genes were defined based on bioinformatic analyses of genome sequences or based on sequence homologies to essential genes or virulence factors from known model organisms. That document does not contain any indications of the genes disclosed in the present invention for the generation of a resistance against an oomycete of the genus Phytophthora. 
A method for producing a broad spectrum resistance in transgenic plants against multiple fungi is described in WO 2009/112270. In one implementation of the method of that invention, the broad spectrum resistance is directed against Uncinula necator, Plasmopora viticola, Uromyces spec., Phakopsora pachyrhizi, Erysiphe sp. and also P. infestans. 
Furthermore, the development of Phytophthora infestans-resistant potato plants through RNAi-induced silencing is disclosed in WO 2006/047495. On the one hand, plants were generated which carry gene sequences of the rRNA gene from Phytophthora infestans for RNA interference. The silencing construct described in WO 2006/047495 directed against the rRNA gene of Phytophthora infestans comprises base pairs 1-600 of Accession number AJ854293 and with it 32 bp of the coding region of the 18S rRNA as well as the complete coding region of the 5.8 S rRNA gene of the blight pathogen. When selecting the target genes for HIGS strategies, with a view to applicability, it is vital that it has as short as possible or preferably no homologies extending over more than 17 sequential base pairs to the gene sequences of non-target organisms, as if there were, gene expression of the non-target organisms in the case of consumption of the transgenic plant or its harvest product could be destroyed (“off-target” effect). However, the sequence described in in WO 2006/047495 comprises 32 bp of the P. infestans 18S rRNA, which has 100% identity with the homologous sequence of the 18S rRNA gene from man (Homo sapiens), pigs (Sus scrofa) and cattle (Bos taurus). Human potato consumption in Asia in 2005 was 26 kg, in North America it was 58 kg and in Europe it was 96 kg per person (FAOSTAT). In the light of the high human and animal consumption of potatoes, the rRNA sequences from Phytophthora infestans described in WO 2006/047495 as HIGS target genes are unsuitable for consumers on safety grounds.
On the other hand, in WO 2006/047495, plants were produced that carry gene sequences for the cathepsin B gene from Myzus persicae and the elicitin gene INF1 from P. infestans for RNA interference and thus exhibit resistance to two plant pathogens. The target gene INF1 used therein codes for an elicitor. A resistance based on an elicitor as a pathogenicity factor is a disadvantage because the elicitin gene INF1 is not always necessary for an infection of potatoes by Phytophthora infestans (Kamoun et al. 1998).